I tried every radnom (3,7) seconds, every choose (3,4,5,6) seconds, and every private variable X = random (3,7), there is no way to set something every radnom seconds with Every x seconds expresion...You must triger it by calculating time of action(do something), curent time, and random time period.

Do something if curent time=action time+radnom time, then set action time to curent time, set new radnom time period...